[Bug target/105075] [nvptx] Generate sad insn (sum of absolute differences)

["tschwinge at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105075

Thomas Schwinge <tschwinge at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu.org,
                   |                            |tschwinge at gcc dot gnu.org
   Last reconfirmed|                            |2022-03-28
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #4 from Thomas Schwinge <tschwinge at gcc dot gnu.org> ---
(In reply to Tom de Vries from comment #0)
> ptx has sad ((sum of absolute differences)) insn, which is currently not
> modeled in the .md file.

I also had noticed this, a while ago (but no opportunity to work on it
properly).

I had found GCC/RTL 'ssad', 'usad', and 'gcc/tree.def:SAD_EXPR', but not
verified their applicability.


(In reply to Tom de Vries from comment #2)
> AFAIU, at gimple level support is limited to vectors, so that doesn't help
> to generate the insn for the simple, scalar case.
> 
> It would be nice if combine could generate it and we wouldn't have to use a
> peephole, but AFAIU the pattern is too complex for that.
> 
> I wonder if reformulating using a conditional could help there (ptx isa
> describes semantics using "c + ((a<b) ? b-a : a-b)".

Would that not be a case for a proper TREE-level representation, and
'gcc/match.pd' patterns to catch and transform any appearances to that, and
then let each back end generate an ideal sequence of instructions (with a
default to the expression you gave above)?

> Also, I wonder if defining a stepping-stone intermediate pattern could help
> combine.